1. Field of the Invention
This invention relates generally to the field of geophysical exploration. More specifically, the invention relates to a method of patterned seismic shots for marine applications.
2. Background of the Invention
In marine seismic surveys, a seismic energy source is used to generate seismic energy in the form of acoustic pulses or waves in a body of water such a lake or the ocean. The seismic energy travels downward in the water, through the water bottom, and through the subterranean formations underlying the water bottom. Part of the energy passing through the subterranean formations underlying the water bottom is reflected upward therefrom at acoustic impedance boundaries in the Earth formations. The upward traveling energy is detected by sensors such as hydrophones towed in one or more streamer cables disposed near the water surface, or by sensors disposed in cables along the water bottom. The sensors convert the detected energy to electrical or optical signals. The electrical or optical signals are then conditioned and interpreted to provide information both as to the composition and the structure of the various subsurface Earth formations. Such information is used particularly to determine the possibility that such Earth formations may contain mineral deposits such as hydrocarbons.
Several different types of seismic energy sources have been used in the past to produce seismic energy in a form required in marine seismic surveys. For example, explosives have been used as a marine seismic energy source. Another type of marine seismic energy source, called a gas gun, includes detonating combustible gases in a chamber and then expelling the resultant gas charge into the water to produce the seismic energy. In marine seismic data acquisition, an air gun is the most employed acoustic or seismic source. In such air guns the sound is generated by letting high pressure air (100-200 atmospheres) escape through port openings in the air gun.
A single air gun produces a seismic pulse having acoustic energy related to a complex pressure interaction between the air bubble and the water that causes the bubbles to oscillate as they rise toward the water surface. Such interaction can produce extraneous bursts of seismic energy following the initial energy burst. The amplitude and periodicity of these bubble-generated extraneous bursts depend on, among other factors, the depth of the gun in the water and the size of the pressurized air chamber in the gun. As such, it is common to use an array of air guns having various different chamber sizes, and firing such guns simultaneously. Such firing of an array of air guns provides several advantages over firing a single air gun. First, the total amount of energy being imparted into the Earth's subsurface for each seismic “shot” is increased. In addition, the different chamber sizes for the various guns will produce different bubble responses, causing the bubble responses to tend to cancel each other. The directivity of the energy source toward the water bottom can be improved, because other than directly below the source array, some frequencies in the seismic energy will be attenuated by the spatial distribution of the guns in the array. Thus, conventional air gun arrays simultaneously discharges all the air guns in the array. This generates a strong signal with a more impulsive signal than any single air gun.
The design of conventional marine air gun arrays is usually fixed for the duration of a seismic survey. While some air gun array attributes can be altered in processing, more processing flexibility in shaping the source signal is desirable. Conventional air gun arrays generate a strong impulsive signal (see FIG. 5). In addition to generating a seismic signal into the subsurface the impulse may create interfering noise with other seismic surveys, mechanical vibration on the hull of the towing vessel and fatigue for the crew.
Consequently, there is a need for a method and system for seismic acquisition that will allow the reconstruction of a seismic source array in processing enabling more flexibility than a conventional fixed air gun array, while maintaining signal strength.